Fix DCE of unreachable phi in cycle

We can't remove a trivial phi of the form x = phi(x), because we
don't have a replacement value. We could drop the whole block
though. SCCP would

Fix DCE of unreachable phi in cycle

We can't remove a trivial phi of the form x = phi(x), because we
don't have a replacement value. We could drop the whole block
though. SCCP would normally do this, but in this particular case
we only determine non-reachability based on type information.

Fixes oss-fuzz #39316.

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